The invention relates to a method for preparing a protein product with one or more improved functional properties. The invention further relates to a product comprising such a protein product and to the use of a protein product in a food product.
The proteins of vegetable material such as potatoes and other vegetables that are primarily cultivated as a source for starch have traditionally been regarded as a waste product. However, such proteins may be economically interesting alternatives to proteins from animal origin and their nutritional quality is competitive.
For the recovery of proteins from a juice generated during the starch production, heat coagulation is commonly regarded as an attractive method, at least from an industrial point of view. In “Ernährung Vol. 2, nr. 9, 1978, by Knorr and Steyrer”, a method is described wherein proteins are recovered from an acidified juice that is heated to a temperature of approximately 98-99° C. The protein fraction is thereafter concentrated by centrifugation. Next, the pH of the fraction is adjusted to pH 7, after which the product is dried by freeze drying, spray drying or drum drying. It was found that the water binding properties were least reduced in a potato protein concentrate that had been freeze dried.
In “Lebensm. Wiss. u. Technol. 13, 297-301 (1980)”, Knorr reports the effect of the pH prior to drying on the water binding properties of a potato protein product that has been obtained by acidic and thermal coagulation. The pH of the protein coagulate is neutralised to pH 7 with NaOH, prior to a drying step. The neutralised protein coagulate is freeze dried or spray dried and ground. The publication shows water-binding properties of the resulting protein products within the range of pH 5-7. Depending upon the exact process, the effect of the pH varied from a marginal increase to a considerable decrease in the water binding capacity. The results do not indicate a trend of a particular dependency of the water binding capacity on the pH value, so it is not possible to predict an effect of the pH upon the water-binding of a protein product. As far as an improvement may be realised, such an improvement is not sufficient, in particular not for demanding applications.
In Lebensm. Wiss. u. Technol. Vol. 5 (1972) no. 1, pp. 24-29, A. M. Hermanson reports about food swelling properties of three protein products. The protein products have a solubility in water of 55-90 wt. % and give rise to a neutral pH (6.5-7.4) in water. A 10% dispersion in an aqueous solution of each protein product was gelled—after sealing the dispersions in cans—by heating the dispersions for 30 min. Thereafter, the samples were cooled in an ice bath. The effect of pH is shown by using an aqueous buffer solution comprising glycine-NaOH or K-biphtalate-NaOH. Sodium-caseinate showed a much higher increase in fluid uptake with increasing pH than soy bean protein isolate or whey protein isolate. For the soy-bean protein isolate, the swelling behaviour was consistent with a high viscosity, whereas the whey protein product, having a much lower viscosity, did not maintain its water when it was pressed. Further it was reported that a higher ionic strength negatively influenced the swelling, in particular for the soy bean.
According to WO 97/42834, heat coagulation of proteins from potato juice causes the proteins to denaturate, and as a consequence the proteins lose their functional properties, i.e. a loss of emulsifying capacity, foaming capacity, thermogelling capacity, water binding capacity. Even the most essential requirement for its application in the food industry—solubility in, water—cannot be met. Therefore, in WO 97/42834 it is proposed to recover proteins from potato juice in non-denaturating conditions. This is achieved by concentrating the protein by subsequently subjecting the juice to disc stack centrifugation, ultrafiltration, diafiltration and optionally freeze-drying. Such a method is however laborious, relatively expensive and special precautions with respect to the temperature have to be taken in order to avoid denaturation. In addition, a method making use of ultrafiltration and the like, may be disadvantageous with respect to the microbiological controllability.
The loss of functional properties of a protein product is undesired in many applications. Loss of water-binding or fat-binding capabilities of a protein product may for example, make the use of a protein product in a food product less attractive, because high water- and/or fat-binding properties may contribute to avoiding curdling or another form of phase separation in a food product. Even if the water-binding or fat-binding capabilities are satisfactory, several known products give rise to an undesired flavour or odour when processed further, making them less suitable for food applications. Other drawbacks of known protein products include pricing of the raw materials and a high viscosity.
It is an object of the invention to provide an economically attractive method that allows the production of a protein product with satisfactory functional properties. It is further an object of the invention to improve one or more functional properties of a vegetable protein product in comparison to products comprised in the state of the art.